Quote terminal attribute value for unary slash elements

Fixes #233
In this PR, we retain quotes around the ultimate attribute for any element, regardless of the attribute's name.

For example, if we have an element like this.
```
<img data-foo="bar" />
```

It would be inappropriate to minify it thus

```
<img data-foo=bar/>
```

In this case, the data attribute value would come back as `"bar/"`.

So it's not about quoting based on any specific attribute name, but instead based on attribute position.

Alternatively, we could just emit an extra space before the closing unary slash for elements with attributes

```
<img/>
<img data-foo=bar />
```

Not sure why I didn't think of that before :)